The invention relates generally to driving a carrier head in a wafer polishing system.
Wafer polishing techniques, such as chemical mechanical polishing (CMP), are used to planarize the surface of a semiconductor or other wafer. One or more layers previously may have been formed on the surface of the wafer. CMP techniques, for example, typically include mounting the wafer on a carrier or polishing head. The exposed surface of the wafer is placed against a rotating polishing pad. The carrier head provides a controllable load, in other words pressure, on the wafer to push it against the polishing pad. A polishing slurry is supplied to the surface of the polishing pad.
The effectiveness of a CMP process can be measured by its polishing rate, and by the resulting finish (absence of small-scale roughness) and flatness (absence of large-scale topography) of the wafer surface. The polishing rate, finish and flatness are determined by the pad and slurry combination, the relative speed between the wafer and pad, and the force pressing the wafer against the pad.
Various non-uniformities in the polishing process can adversely affect the quality of the polished wafers. Such non-uniformities may result from changes in the condition of the polishing pad. For example, the pad may become glazed in regions where the wafer was pressed against it. Such a condition may cause parts of the pad to become less abrasive and can result in the polishing process varying from one wafer to the next.
In general, a wafer polishing apparatus includes a carrier head having a central axis and a drive shaft coupled to the carrier head. A first input pulley is coupled to the drive shaft to drive the carrier head about its central axis. A second input pulley is coupled to the carrier head to drive the carrier head in a circular path about a point that is offset from the central axis of the carrier head. A controller can be provided to regulate the speeds of the pulleys.
In various implementations, the apparatus can include one or more of the following features. The controller can be operable to cause the carrier head to move in the circular path while the carrier head rotates about its central axis. The controller also can be operable to cause movement of the carrier head in the circular path at a speed that is independently controllable from the speed at which the carrier head is rotated about its central axis. Furthermore, the controller can be operable to cause rotation of the carrier head about its central axis and movement of the carrier head in the circular path while the wafer is held in contact with a polishing pad that may be positioned, for example, on a platen. Various details of the apparatus and its operation are described in greater detail below.
In a related aspect, a method of polishing a wafer includes holding the wafer in a carrier head having a central axis, bringing the wafer into contact with a polishing pad. When the wafer is in contact with the polishing pad, the carrier head can be rotated about its central axis and simultaneously, the carrier head can be moved in a circular path about a point that is offset from the central axis of the carrier head. The carrier head can be moved in the circular path at a speed that is independently controllable from the speed at which the carrier head is rotated about it central axis.
In other implementations, the system of input pulleys and corresponding output pulleys coupled by belts can be replaced by input gears and corresponding driving gears.
Various implementations can include one or more of the following advantages. Rotation of the carrier head about its own axis can impart or enhance the relative motion between the polishing pad and the wafer. Additionally, rotation of the carrier head about a point that is offset from the carrier head""s axis can sweep the carrier head across the larger area of the polishing pad. The sweeping motion of the carrier head across the pad can help randomize non-uniformities in the pad and can reduce the amount of wear to the pad. The techniques can be used in situations in which the polishing pad is stationary as well as when the pad is rotated.
Other features and advantages will be apparent from the following description, the accompanying drawings, and the claims.